Does Fitech offer a way to view datalogs graphically? I gave the megasquirt software a try and it works, but only 500 data points at a time with the free version.
https://www.efianalytics.com/MegaLogViewer/
I think your O2 sensor is probably ok. It seems to respond appropriately to fueling pulsewidth. One change I would make is to richen up your target AFR at idle to something in the 13.5-14.0 range. It will make the idle and afr more stable. I can see in the data your idle afr jumps up and down quite a bit even with small changes in pw. I had the same issue with my megasquirt setup, trying to idle at stoichiometric afr. 13.5 worked a lot better for me, and helped transient throttle tip in too. Plugs stay clean as long as the engine is up to temperature and driven/loaded.
I think you issue isn't fuel related, but probably spark or valvetrain or something else. The screenshot below of your datalog I think shows the issue occuring. You have the throttle opened to 14% and then 17%, but the engine wont rev above ~1500 rpm. AFR looks fine here so I don't think fueling is the issue. You should be able to rev it to 1500 with small throttle opening, and the manifold pressure should drop (engine VE better at higher rpm), but it's actually increasing.
I'd go through the ignition system and inspect everything. What is your specific ignition setup? How is the Fitech triggered, and how does it control the coil?
I'd also do a compression or leakdown test, and check the valvetrain, rocker adjustment, valve lift, etc.
Maybe someone with Fitech experience will catch something I missed